<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        #box{
            display:flex;
            flex-direction: row;
            flex-wrap: wrap;
            justify-content: space-around;
            align-items: center;
            width:906px;
            height:906px;
            border: 1px solid red;
        }
        .innerbox{
            width: 250px;
            height:250px;
            background-color: green;
            /* border: 1px solid red; */
        }
    </style>
</head>
<body>
    <div id="box">
        <div class="innerbox"></div>
        <div class="innerbox"></div>
        <div class="innerbox"></div>
        <div class="innerbox"></div>
        <div class="innerbox"></div>
        <div class="innerbox"></div>
        <div class="innerbox"></div>
        <div class="innerbox"></div>
        <div class="innerbox"></div>
    </div>
</body>
</html>